Type 1 Diabetes (T1D) is an autoimmune disease resulting in the destruction of pancreatic islet insulin- producing beta cells. T1D is mainly drive by autoreactive T cells but components of the innate immune system have been implicated as well, including monocytes, dendritic cells and natural killer (NK) cells. NK cells belong to the innate lymphoid cell (ILC) family, which consists in innate cells that share a lymphoid morphology and other properties. Novel populations of ILCs have been defined recently and ILCs are now subdivided in three main categories: ILC1 cells (NK cells); ILC2 cells (IL-13- and IL-5-producing GATA-3+ innate helper cells, and ILC3 cells (ROR?t+ ILCs, which include distinct subsets of IL-22- and/or IL-17-producing ILCs). Functionally, ILC2s and ILC3s are involved in protective immunity against infections but they can also induce or control chronic inflammation depending on the circumstances. Importantly, the role of ILCs in autoimmunity is largely unknown. Thus, in this proposal we will address the overall hypothesis that ILC populations play a critical role in the development and progression of autoimmune diabetes. We will take advantage of innovative mouse models that include reporter mice allowing the identification and deletion of ROR?t+ ILC3s and IL-5/13-producing ILC2 cells selectively. Most significantly, we observe that ILCs are the first cells found in the pancreas of T1D-susceptible non-obese diabetic (NOD) mice suggesting a potential role in disease initiation. Finally, we will address another key hypothesis, namely, that ILC2 cells, present in tissues, including the pancreas, are critically involved in the negative side effects of IL-2 therapies designed to promote regulatory T cells (Tregs). Recent studies to test the safety of IL-2 therapy in new-onset T1D patients have validated mouse work demonstrating increased numbers of Tregs..However, in some instances, the treatment resulted in transient ? cell dysfunction, increased Th17 cells and eosinophilia. Importantly, our preliminary studies suggest that ILC2s present in the pancreas of NOD mice respond to IL-2 treatment and may be responsible for some of these unwanted effects. In this R21 Revised application we propose the following specific aims to address these hypotheses. 1) To characterize the ILC2 and ILC3 subsets in the NOD mouse; 2) To determine the functional effect of ILC2s on the development and regulation of autoimmune diabetes; and 3) To examine the effect of IL-2 therapy on ILC2s and the role of ILC2s in the biological and clinical effect of IL-2 therapy. These studies will characterize novel ILC subsets i the pancreas during diabetes and identify their influence on disease. To our knowledge, this will be the first characterization of ILC2s and ROR?t+ ILC3s in autoimmune diabetes. The results of our study may shift the current T1D dogma from a T cell-centric paradigm to one that includes an involvement of ILCs. Additionally, our studies could have important clinical implications by identifying novel therapeutic targets in T1D and allowing better a prediction of off-target effecs of therapy directed at pathways shared by ILCs and T cells.